Designers and assemblers of radial ball bearings have always faced the dilemma that, in the completed bearing, the radial clearance between the ball pathways of the races is less that the diameter of the bearing balls. In order to assemble the balls, therefore, some means is necessary to temporarily, or locally, enlarge the radial clearance.
Conventionally, this has been done in one of several ways. One is the familiar Conrad technique in which the races are eccentrically displaced, and the balls pushed through the localized area of larger clearance. Then, the races are moved back to a concentric orientation. This has the advantage that the ball pathways are completely integral to the races, with continuous, uninterrupted surfaces, but there is a substantial limit on the number of balls that can be installed. Another technique is to cut a localized loading slot into the ball pathway, which is somehow plugged or filled after the balls are loaded in. This allows a higher ball count than the Conrad technique, but leaves a permanent physical interruption and stress riser in the pathway.
Since load capacity and ball count generally go hand in hand, and since both load capacity and axle stiffness are critical in vehicle wheel bearings, practical designs for wheel bearings are almost always double row angular contact ball bearings. The larger total number of balls gives higher load capacity, and the axial separation or "straddle" of the two rows gives axle stiffness. In this type of bearing, at least one pair of pathways, the inner or the outer pair, or sometimes both pairs, do not wrap both sides of the ball, as in a full contact radial bearing. Instead, the pathways wrap only diagonally opposed sides of the balls in each row, creating imaginary "contact cones". These cones may diverge, and so not intersect externally of the bearing, or converge, and thereby intersect externally of the bearing. It is generally desirable for loading and handling considerations to have a convergent wheel bearing design. When building up the bearing, it is a simple matter to get a high ball count in the ball row that is first installed, since there are no obstructions at that point in the assembly process. In installing the last ball row, however, the same problem of ball access is faced as with a single row bearing, since the inner and outer races (called spindle and hub) are then both in place. The Conrad technique is obviously more difficult to apply in the double row application, though not impossible, but generally does not give a high enough ball count. The loading groove technique is unacceptable because of the stress riser. Therefore, what has generally been done is to provide an entirely separate race piece for the last ball row, which gives clear, unobstructed ball loading access. The separate race piece is then locked axially in place by a control ring, which also determines the ball preload. The patent covering this design and technique is U.S. Pat. No. 4,179,167 to Lura, co assigned.
What is not articulated in the Lura patent, but what is obvious with some analysis, is that the technique disclosed there will work only in the case of a convergent double row design with a separable inner pathway, or a divergent design with a separable outer pathway. The technique will not work in the case of either a convergent design with separable outer pathway, or a divergent design with a separable inner pathway. This is because, in those two cases, the separable piece would have to make rolling contact behind the last installed ball row, and would be too thick to be capable of being pushed axially through the available radial clearance. While divergent wheel bearing designs are generally not desirable per se, a convergent design with a separable outer pathway would have some advantages. The outer pathway is larger in diameter than the inner, and can take more load. In the case of live spindle wheel bearing, that is, one in which the wheel is mounted to a rotating spindle, it would be an advantage for the separable piece to be fixed to the outer, stationary hub. Co assigned U.S. patent application Ser. No. 08/072,378, filed Jun. 7, 1993, discloses a novel design that does allow a convergent, separable outer pathway wheel bearing to be assembled. The separable outer race is designed to be pushed into the hub first, before the second ball row is installed, and retracted to an assembly position that opens up sufficient ball loading clearance. The outer race is then pulled back and locked in place to the stationary hub by a control ring, the thickness of which sets the ball preload.
Another consideration faced by vehicle wheel bearing designers is the pressure to continually simplify designs in terms of numbers of parts and package size. A trend has been to integrate more and more parts together, so that the bearing ball pathways become integral parts of the wheel spindle and hub. A downside of this trend is that bearing pathways require very high quality steel, more so than would be absolutely required for other parts of the wheel spindle and hub. The Lura patent takes this integration about as far as it has been commercially practicable to do. The pathways are integral to the hub, but the hub itself is separately bolted to the suspension, rather than being integral to the suspension. It is bolted to the outboard side of the suspension member, meaning that the bearing has to fit into whatever axial space is available between the suspension member and the wheel flange, which limits the width of ball row straddle available. The next desirable step in integration would be to integrate the outer wheel bearing hub with the suspension member, rather than separately bolting it on. In the case of a front suspension, this would involve integrating the hub and outer pathways to the steering knuckle, which is a very large piece, and would be prohibitively expensive if it were made entirely of bearing quality steel. Therefore, proposals for integration of hub and steering knuckle typically use separable races to provide the actual pathways. An example may be seen in FIG. 6 of U.S. Pat. No. 2,631,865 to Hoffman. The steering knuckle and hub are integral and compact, but integrating the hub makes the steering knuckle a much more complex part. Furthermore, the hub surfaces against which the inserts are fitted are fairly deep inside the hub, and not easily accessible for precision grinding. Consequently, the design is not particularly practical, and has not found commercial acceptance.